A recent development for the in-mold addition of treating agents within the molten metal for castings is disclosed in a co-pending U.S. Patent Application Ser. No. 488,758, filed July 15, 1974 by John R. Nieman et al and entitled METHOD AND APPARATUS FOR THE INTRODUCTION OF ADDITIVES INTO A CASTING MOLD concurrently filed herewith by the Assignee of the present application. The co-pending application teaches continuously feeding a wire of additive material into a stream of molten metal by extending the wire directly into the pouring basin and down sprue within the casting mold. The down sprue acts as an orifice for controlling the flow rate of the molten metal into the casting cavity and the feed rate of the wire is selectively adjusted to provide precise control over the amount of treating agents added to the casting. Although such method is a significant improvement over currently available in-mold methods of adding treating agents, it is recognized that further improvement could be obtained by forming an additional mixing chamber within the casting mold specifically for the purpose of simultaneously receiving the wire of additive material and the flow of molten metal to insure a more uniform dissolution of the treating agents into the molten metal.